. Bulletin of the British Museum (Natural History), Geology. 150 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS Description. This form occurred in moderate abundance in the upper levels of the Cenomanian of the Fetcham Mill Borehole, over 20 specimens having been seen ; the apex was missing in all specimens seen. The shell is ovoidal in shape, with a somewhat flattened antapex and an apex truncated by the archaeopyle. Six precingular and six postcingular plates are present. The presence of an anterior intercalary plate was suggested in some specimens, but could not be confirmed as a result of dis
. Bulletin of the British Museum (Natural History), Geology. 150 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS Description. This form occurred in moderate abundance in the upper levels of the Cenomanian of the Fetcham Mill Borehole, over 20 specimens having been seen ; the apex was missing in all specimens seen. The shell is ovoidal in shape, with a somewhat flattened antapex and an apex truncated by the archaeopyle. Six precingular and six postcingular plates are present. The presence of an anterior intercalary plate was suggested in some specimens, but could not be confirmed as a result of distortion around the archaeopyle. Plates i'" and 2'" are reduced to accommodate a large posterior intercalary plate ; this is of comparable size to the single antapical plate, some specimens indeed give the impression of having two antapical plates. The cingulum is very broad and not hollowed ; it is weakly spiral, laevorotatory, its two ends scarcely differing in antero- posterior position. The sulcus is broad throughout its length, but broadest as it approaches the antapex : ventral plates are absent. The shell surface is generally quite smooth, but bears a scatter of tubercles, some of which are aligned parallel to sutures ; the number and arrangement of tubercles varies between individuals. The sutures bear closely set spines, capitate and of constant length, giving almost the impression of perforate crests : in vertical view, these give the impression of a string of beads. Remarks. These English Cenomanian forms generally closely resemble Micro- dinium ornatum as described from AustraUa, but differ in two details—the absence of a plate separating the posterior end of the cingulum from the sulcus, and the form of the crests, which are constantly in the form of closely set spines. It is highly probable that they fall within the range of variation of M. ornatum, since they accord closely with the photographs and since Cookson & Eisenack state (p. 7) that "
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